Field of the Invention
This invention relates to X or Gamma indirect image detectors that incorporate a fiber optic plate (FOP) to transfer visible light to the imaging sensor, and more particularly to a structure and method of assembling of the FOP to reduce shear stress and the possibility of localized delamination that would produce artifacts in the detected image.
Description of the Related Art
With indirect flat image detectors, the X or Gamma ray radiation penetrates through an object to be examined and encounters a scintillator layer that converts the X or Gamma ray radiation into visible light. If CCD or CMOS imaging sensors are used to detect the light, a fiber optic plate (FOP) is used as an intermediate layer. The FOP allows the converted visible light to pass through but blocks the X or Gamma radiation and thus protects the sensitive sensors. The FOP consists of many individual optical fibers aligned in parallel through which the light is guided. The FOP transfers an image from one end of the fiber to the other without any distortions.
The detector is assembled by forming a coupling layer of adhesive or coupling oil approximately 5 to 100 microns thick on the surface of the imaging sensor and then directly contacting the FOP to that coupling layer. The adhesive or coupling oil may contain small glass spheres that act as spacers. The spheres are intended to provide a defined space, following the macro non-flatness of the FOP and imaging surfaces. The adhesive or coupling oil are suitably index matched to reduce optical distortion and maintain the modulation transfer function (MTF). If the coupling layer is too thick, the optical losses will reduce the MTF.
The coupling layer serves to physically attach the FOP to the imager. This creates shear stress due to TCE (Thermal Coefficient of Expansion) mismatch between the FOP (5-8 ppm/C) and the imaging sensor (approximately 3 ppm/C). The shear stressses may cause localized delamination of the adhesive or coupling oil (air pockets) producing artifacts in the detected image. The coupling oil produces less shear stress but is disfavored as it may contaminate the entire detector and tends to dry out over times.
To maintain the detector's MTF, the thickness of the coupling layer is on the order of the surface flatness of the imaging sensor (e.g. the Silicon). The imaging sensor may have a specified peak-to-valley variation of, for example, 55 microns and the coupling layer may be nominally the same thickness. When the FOP is mounted onto the coupling layer, the layer may be very thin at certain points and the FOP may be in direct contact with the surface of the imaging sensor at its highest peaks. Direct contact of the FOP to the active pixels (imaging area) or amplifier circuitry can damage those components. These points have a higher stress, and thus higher risk of delamination as well. The use of the spherical balls as spacers is typically ineffective as they tend to cluster in the valleys on the surface of the imaging sensor and thus do not effectively space the FOP from the imaging sensor. These clusters also tend to inhibit flow of the adhesive or coupling oil and can produce optical artifacts. The glass spheres may also reduce product life time as they can cause point stresses in the detector.